The present invention relates to a drive control method, namely a drive control method for a heavy-duty drive, in particular a heavy-duty drive of a vertical mill designed for crushing brittle materials, for example raw cement material, and a corresponding drive system operating according to said method.
Vertical mills of the type cited above comprising a grinding plate rotating about the vertical axis and grinding rollers above the grinding plate tend to be subjected to high mechanical oscillations, since in simple terms the drive train of a vertical mill is an oscillating system in the form of a dual mass oscillator. The grinding plate and all of the units moved by the grinding plate form part of the first mass and the second mass is the rotor of the driving motor. The connection between these two masses is in the form of a gearing which functions in the manner of a torsion spring in the oscillating system. The system is excited to oscillate briefly or even for longer periods of time by a continuous low-frequency load change as a result of the grinding process and occasional varying loads as a result of the grinding process. The resulting forces and torques may be so high that the grinding process has to be stopped in order to avoid damage to the drive train, namely in particular to the electric motor and/or to the gearing or the system as a whole. The reason for the echoing of oscillations in the drive train (drive train oscillations) is that the damping in the drive train (drive train damping) is not sufficient in order to compensate for the oscillation energy introduced from the working machine (grinding machine).
In order to keep such oscillations low, the operator of the mill previously had to design the process parameters, i.e. in particular the contact pressure of the grinding rollers, the composition of the grinding material and the quantity of additional grinding aids, such that the excitation of oscillations remained below a critical level. This leads, however, to undesirable restrictions in the configuration of the process, having a negative effect in many areas. For example, the range of products which are able to be produced by the grinding material which has been respectively obtained, the efficiency of the mill, the required energy use and the cost efficiency are affected. Moreover, such a procedure is highly unreliable as a great deal of experience is required in order to carry out the process correctly and the properties of the ground natural materials are always variable. Thus it repeatedly results in greater torsional oscillations.
In view of this background and due to increasing requirements with regard to availability, efficiency and service life costs (TCO=Total Cost of Ownership) the design and arrangement of the electrical and mechanical components of a drive system and the respective drive train of a heavy-duty drive, in particular a vertical mill, are becoming increasingly important.
Currently, drive systems with a gearing and at least one electric motor in the form of an asynchronous motor, preferably a slip ring motor, as well as a frequency converter supplying the at least one electric motor, represent a preferred solution for vertical mills. In this case, in practice the mill gearings are frequently designed as variants of planetary bevel gear transmissions or planetary spur gear transmissions. The object of the gearing is to absorb the axial grinding forces and to transfer said forces into the substructure, in addition to the rotational speed conversion and torque conversion.
Hitherto, attempts have been made to solve the aforementioned problem by integrating a mechanical element into the drive train, wherein the respective mechanical element or optionally a plurality of mechanical elements is and/or are characterized by a correspondingly high damping effect. Currently couplings, in particular highly resilient elastomer couplings, are used as such elements with a sufficiently high damping effect.
A drawback with this solution is firstly that such a coupling is an expensive wear part. Secondly, the drive train damping achieved by a coupling is implemented by the conversion of oscillation energy into thermal energy which has a negative impact on the energy footprint. Finally, the degree of drive train damping which is able to be achieved by such a coupling is still very small and has been shown to be insufficient in some cases.